Stranded electrical conductors fabricated with a plurality of round wires made of an electrically conductive metal, such as copper or aluminum, are well known in the art, as are methods and apparatus for making these stranded conductors. Such conductors are customarily fabricated by stranding together a plurality of wires in concentric layers about a core wire. As used herein, the term "core wire" includes a single core wire as well as a stranded conductor used as a core wire for a second or subsequent layer of wires. The natural geometry of such a construction is that when round wires of the same diameter are used to form a stranded conductor, six wires naturally fit around a single core wire of the same diameter, twelve wires fit naturally around the layer of six wires, eighteen wires fit around the layer of twelve wires and so on with each successive layer containing six wires more than are contained in the layer around which they are being stranded. Conductors of this configuration are known as concentric lay conductors. The number of individual wires contained in any conductor having "n" layers of wire about a core wire of a common diameter is calculated by the algebraic equation X=6(n)+1; with "X" being the number of wires in the conductor and "n" being the number of layers of wire about the center or core wire.
Generally speaking, there are three conventional types of apparatus for making stranded electrical conductors which have a plurality of round wires twisted about the longitudinal conductor axis. One apparatus, known as a rigid frame strander, employs a rotating pay-out system. In a rigid frame strander, a plurality of spools of wire are mounted on a rotatable laying head through which a core wire passes. As the laying head is rotated, the wires from the plurality of spools are helically wrapped or twisted about the advancing core wire and passed through a closing die to form a stranded conductor which is then collected on a take-up reel or bobbin. One of the main disadvantages of this type of strander is the slow speeds at which the apparatus must be operated.
A second type of apparatus employs a rotating take-up reel in which the take-up reel is rotated about two axes, namely, the reel axis for take-up purposes and the conductor axis to provide twists to the conductor. In this second type of apparatus, a plurality of wires are advanced in substantially side-by-side relation from a plurality of spools or stem packs mounted on a stationary platform. The wires are guided to a stationary lay plate. One of the wires passes as a core wire and the remaining wires are concentrically spaced about the core wire. The wires are passed from the lay plate to a closing die and thence to a take-up reel which twists the stranded conductor.
The third known type of apparatus for making stranded cable is a strander, e.g., a double twist strander, in which the wires are advanced from stationary spools in side-by-side relation through a stationary twist plate and to a closing die. In the strander, however, neither the pay-out system nor the take-up system rotates about the axis of the conductor. A twist is applied to the wires of the stranded conductor by a rotating bow mechanism located between the closing die and the take-up reel. Advantageously, the double-twist strander is a more efficient and economical apparatus than either the rigid frame strander with a rotating pay-out system or the apparatus with a rotating take-up reel because the double twist strander provides two twists in the stranded conductor for each revolution of the rotating bow. Thus, for a given speed of rotation, the production rate of a double twist strander is almost twice the production rate of the machines with a rotating pay-out or take-up system. Moreover, the double twist strander is a more compact system because the pay-out spools and the take-up reel need not be mounted for rotation as they must in other types of stranding apparatus.
Of primary concern when forming a stranded conductor on a double twist strander is the need for uniform tension on the stranded conductor as it is being collected on the take up reel. Uniform tension is required to prevent any of a number of undesirable events from taking place.
Absent adequate and uniform tension, a conductor bunched and then twisted by the double twist strander will contain wires that do not lay substantially flat about the core wire. This condition is known as a "high wire" in the conductor. This high wire cannot be properly insulated, nor will it maintain its position in the conductor if the conductor is used bare. High wires spawn a loose cable configuration that will not maintain its lay during use.
Inadequate and non-uniform tension on the conductor being collected also contributes to a condition known as "cross over". Cross over occurs when the conductor is placed on the reel and a previously placed wrap of wire slides across the layers of wire and crosses over the top of the wraps subsequently placed and tension is then applied. This condition results in a binding of the latter wrap by the previous wrap. When attempting to remove the conductor from the reel, tangles will result at the point where the cross over is found. Additionally, if sufficient tension is applied when paying off the conductor, the binding at the cross over can actually contribute to plastic tensile deformation, thereby resulting in neckdowns in the cross section of the conductor. In extreme cases, the conductor may actually break from the tension at the cross over.
Another advantage of adequate and uniform tension is that the wire can be "even wound" about the reel or bobbin. This is especially necessary when the stranded conductor is to be removed from the reel by "flipping". Flipping consists of laying the reel on one of its two flanges. The wire is paid off the bobbin as it flips off the arbor and around the top flange. If the reel was filled with conductor having non uniform or inadequate tension, the wraps will be loose and will prematurely release and fall about the arbor near the bottom flange. As wraps fall, they cross over other wraps and the problems associated with cross over, as set out above, occur.
Typical industry practice is to apply back tension to the conductor as it is being collected on the take-up reel or bobbin. This tension is typically provided by some type of resistance clutch driving the take-up. The disadvantage to using resistance clutches is that they are generally incapable of precise adjustment and even less capable of continuous adjustment as the conductor is being formed and collected and the tension requirements change. As a result, most clutches are adjusted so that they provide suitable tension for a full bobbin or reel. With the tension so adjusted, the tension is too great when the bobbin or reel is near empty.
It is this need to provide continuously variable, precisely adjustable, tension to the conductor, after it has been twisted and as it is being collected, that is addressed by the present invention.